This invention relates to an optical head device for use in recording optical information on an optical recording medium and/or reproducing the optical information therefrom in a compact disk (CD) device or a like device which will herein be called an optical disk device. The optical recording medium is typically an optical disk, a digital audio disk, or a digital video disk and will herein be termed an optical disk in general. The optical head device is alternatively referred to briefly as an optical head.
The optical head device includes an optical source, an optical detector assembly, and a focussing lens. The optical source is for generating a coherent beam along a main optical axis and is ordinarily a semiconductor laser. The optical detector assembly is usually a photodiode which is partitioned into a plurality of individual optical detectors. The optical detector assembly is used for the purposes which will be described later in the following. The focussing lens is for focussing an input or incident beam on the optical disk along a lens axis of the focussing lens as a converging beam and responsive to an optical beam reflected from the optical disk as a reflected beam for producing an output or excit beam along the lens axis. Depending on the circumstances, the optical source may be regarded as an element which is separate from the optical head device.
In the manner which will later be described with reference to a few of more than thirty figures of the accompanying drawing, a conventional optical head device comprises a beam splitter for causing the coherent beam to pass therethrough along the main optical axis as the input beam and for splitting the output beam into a split beam which travels along a side optical axis forming an angle of 90.degree. with the main optical axis. An additional optical system is used in directing the split beam to the optical detector assembly along at least one additional optical axis.
An optical head device is described in an article contributed by M. G. Carasso et al to Philips Technical Review, Volume 40 (1982), No. 6, pages 151 to 156, under the title of "The Compact Disc Digital Audio System." According to Carasso et al, the split beam is further divided by the additional optical system into two additional beams which are directed to a four-partitioned optical detector assembly.
Another optical head device is described in the Japanese language on pages 19 to 36 of a publication which is called in translation "Techniques for Mastering CD Digital Audios" and was published May 15, 1985, by a publisher named "Seibundo Sinkosya," Tokyo, Japan. The split beam travels towards a four-partitioned optical detector assembly along a sole additional optical axis which is an extension of the side optical axis.
Such an optical head device is bulky and heavy. This adversely affects implementation of an optical disk device which is either compact and light-weight or is operable for an optical disk of a large recording capacity. Moreover, the conventional optical head devices are high-priced because it is troublesome to manufacture the beam splitter and the additional optical system and to adjust them for optimum operation of the optical head device.
On the other hand, efforts have been directed on providing a compact and light-weight optical head device to the scoop effect known in connection with semiconductor lasers. It has, however, been found that the optical disk device becomes unstable when the scoop effect is resorted to. Consequently, recent trends are to suppress the scoop effect in the optical disk devices.